We propose to investigate the biology of steroid-receptor-negative breast cancer, using three approaches: (1) We will fuse steroid-receptor-negative and -positive breast cancer cells to see if steroid-receptor-negativity, and its associated phenotype are suppressible (i.e., recessive genotypic characteristics); (2) We will fuse steroid-receptor-negative nuclei (karyoplasts) with steroid-receptor-positive cytoplasm (cytoplasts) to see if the steroid-receptor-negative nuclei can respond to a 17-B-estradiol in the presence of cytosolic steroid receptor; (3) We will use a monoclonal antibody directed against a steroid-receptor-positive human breast cancer cell line to probe the dynamic relationships of steroid-receptor-negative and -positive breast cancer in vitro and in vivo. Completion of these research projects should significantly increase our knowledge of the biology of steroid-receptor-negative breast cancer, a previously little-investigated problem in experimental oncology.